1. Field of the Invention
The present invention relates to a toner for developing latent electrostatic images to visible toner images in the fields of electrophotography and electrostatic recording, and more particularly to a toner comprising a thermoplastic resin and a coloring agent.
2. Discussion of Background
Because of a recent demand for images with high quality and high durability in electrophotographic systems, it is required that toner particles be minimized in size, and have a sharp particle size distribution, high coloring performance, and uniform charge control performance. A conventional toner for developing electrostatic images in visible toner images is usually produced by fusing and kneading a mixture of a thermoplastic resin and a coloring agent, pulverizing the kneaded mixture, and classifying the pulverized mixture. If toner particles having small particle sizes with a narrow particle size distribution is selectively obtained from the toner produced by the above method, the production performance and yield are significantly decreased, and the cost of such a toner is unquestionably high. Furthermore, the smaller the particle size, the less uniform the dispersion of a coloring agent and a charge controlling agent contained in the toner, and the more frequently the improper charging of the toner occurs. The toner particles obtained by pulverising have many protrusions on the surfaces thereof, so that such toner particles tend to be fused and adhere to the surfaces of carrier particles and also to the surface of a toner layer thickness regulating member. Furthermore, an expensive charge controlling agent that must be present on the surface of each toner particle tends to be included within each toner particle, so that the charge controlling agent does not function properly. If this occurs, the amount of the charge controlling agent must be increased. The result is that the toner becomes costly.
Under such circumstance, many toners which can yield images with high image quality and high durability and methods for producing such toners have been proposed.
For example, Japanese Patent Publications 51-14895 and 47-51830 propose a method of producing core particles, each of which contains a coloring pigment and a charge controlling agent, by suspension polymerization. This method, however, has the shortcomings that a dispersion stabilizer and other agents which tend to adhere to the surface of each core particle are difficult to remove from the surface of each core particle, so that the charging performance of a toner comprising the core particles is caused to deteriorate, and it is difficult to produce core particles, each of which includes the above mentioned coloring pigment and charge controlling agent, with a sharp particle distribution in a stable manner.
Japanese Patent Publications 57-494 and 56-13945 propose a method of producing core particles, each of which contains a coloring pigment and a charge controlling agent, by spray particle formation. This method, however, has the shortcomings that it is extremely difficult to control the particle size, and the produced core particles must be classified, and polymeric resins which are effective for preventing "offset phenomenon" (i.e., a phenomenon that fixed toner images, when superimposed on a plastics sheet, are transferred to the plastics sheet) cannot be used.
For obtaining particles with a uniform particle size, there are known a seed polymerization method comprising the steps of forming seed particles, and causing the seed particles to grow by polymerization, and a dispersion polymerization method of conducting polymerization by using a solvent which is a good solvent for a monomer, but which is a poor solvent for a polymer produced.
The above polymerization methods provide resin particles having small particles sizes with a sharp particle size distribution. However, it is difficult to include carbon black and other coloring agents in each resin particle by the above polymerization methods. In order to remove such shortcomings of the above polymerization methods, varieties of methods have been proposed.
For example, Japanese Laid-Open Patent Application 61-18965 discloses a method of providing a coating layer of a coloring agent or the like on the surface of each dispersion polymer particle by use of a jet mill apparatus. Japanese Laid-Open Patent Applications 63-2075, 63-23166, and 63-244057 disclose methods of embedding pigments into a thermoplastic resin with the application of mechanical strain force thereto. The thermoplastic resin, however, cannot be sufficiently dyed by the methods disclosed in the above Japanese Laid-Open Patent Applications. Furthermore, the fixing performance of the obtained toner is degraded and the offset phenomenon is caused to occur by the presence of the coloring agent on the surface of the thermoplastic resin.
Japanese Laid-Open Patent Applications 50-46333, 61-228458, 63-106667, and 1-103631 disclose methods of dyeing resin particles in a dye solution. When such methods are used, the resin particles can be sufficiently dyed deep into the inside of each resin particle, so that the dye is less present on the surface of each resin particle than in the inside of the resin particle, so that the image fixing performance and "offset phenomenon" preventing performance of the obtained toner particles are excellent. However, a dye which has a sufficiently high dyeing performance and is suitable for the above methods has not yet been discovered.